April 26, 2018

Where the West Begins — The Line is Moving

Driving across the country, I like to play the game of "Where does the West begin?" (westbound) or "Where does the Midwest begin?" (eastbound).

For instance, on US 20 in Nebraska, Valentine is definitely in the West, but anything east of Ainsworth feels like the Midwest.

Driving west across South Dakota, the Missouri River makes an easy marker. From downtown Pierre, I see the dry hills to the west and feel at home. (It helps that I lived as a kid in western South Dakota.)

The aridity gradient [east and west of the 100th meridian] is realized in soil moisture and a west-to-east
transition from shortgrass to tallgrass prairie. The gradient is sharp
in terms of greater fractional coverage of developed land east of the
100th meridian than to the west. Farms are fewer but larger west of the
meridian, reflective of lower land productivity. Wheat and corn
cultivation preferentially occur west and east of the 100th meridian,
respectively. The 100th meridian is a very real arid–humid divide in the
physical climate and landscape, and this has exerted a powerful
influence on human settlement and agricultural development.

This boundary has moved before. An archaeologist friend pointed out to me that in part of the Middle Archaic period (3000–1000 years ago), trees extended farther east onto the plains. Think of of the "pine ridge" country of the Palmer Divide (eastern Douglas and El Paso counties, Colorado) extending clear to Kansas! "Those were the good times," he mused.

It is first shown that state-of-the-art climate models from phase 5 of
the Coupled Model Intercomparison Project generally underestimate the
degree of aridity of the United States and simulate an arid–humid divide
that is too diffuse. These biases are traced to excessive precipitation
and evapotranspiration and inadequate blocking of eastward moisture
flux by the Pacific coastal ranges and Rockies. Bias-corrected future
projections are developed that modify observationally based measures of
aridity by the model-projected fractional changes in aridity. Aridity
increases across the United States, and the aridity gradient weakens.
The main contributor to the changes is rising potential
evapotranspiration, while changes in precipitation working alone
increase aridity across the southern and decrease across the northern
United States. The “effective 100th meridian” moves to the east as the
century progresses.

The Anderson Creek fire burned almost 400,000 acres in Oklahoma and Kansas in March 2016.

Back in the 1930s, the Dust Bowl ripped through the Southern Plains, as plowed land just blew away. So we stopped plowing so much, let it go back to vegetation, and now it's burning. In the long run, that is probably less destructive — more of a natural cycle —but a prairie fire is a scary thing.

In "Why is Oklahoma Burning," weather writer Bob Henson discusses the recent Rhea Fire, which burned more than 242,000 acres.

May 2015 was the state’s wettest single month on record, and 2015 was
its wettest year. “The November-December 2015 period was the wettest on
record as well, and the sixth warmest. So the growing season extended
into winter to some extent that year,” said McManus. The result was an
unusually lush landscape going into the first part of 2016 that dried
out quickly in the weeks leading up to the Anderson Creek fire.

Likewise,
the summers of 2016 and 2017 were on the moist side, said McManus. “We
also had a pretty severe ice storm during January 2017 that left lots of
big fuels on the ground waiting for that spark,” McManus said. Later
that year came the the state’s second-wettest August on record. “August
would normally be a time we'd get rid of some growth in our typical
summer burn season,” said McManus.

The landscape of the Southern
High Plains has been extraordinarily dry over the last six months. The
western third of Oklahoma has seen little more than 2” since
October—only about 20% of average—and most of the Texas and Oklahoma
Panhandles have received much less than 1”, making it the driest six
months on record in some locations. Any moistening of the landscape has
been all too brief, which has left the landscape highly vulnerable to a
spell of fire-friendly weather.

1 comment:

I've been rereading George Catlin's Letters And Notes On The North American Indians, written in 1840, IIRC, from letters as early as 1832. At one point he described fleeing a wild fire in the tall grass prairie, somewhere around the confluence of the Platte and Missouri rivers, I think. He also mentions the difference between fires on short grass and tall grass prairies--those of the short grass are rather leisurely, while those of the tall grass are like hurricanes of fire. The Indians feared them greatly.

Great book. Catlin set out to paint and document the Indian tribes before they were changed forever by the encroachment of whites.